SUMMARY Our long-term objective is to find ways to control methicillin-resistant Staphylococcus aureus (MRSA). Here we focus on characterizing how community-acquired (CA)-MRSA colonizes the gastrointestinal (GI) tract. A key, but underappreciated, observation is that GI colonization establishes a reservoir for transmission and is the most common origin for CA-MRSA infection in infants and young children, who are at greater risk of infection than adults. We and others have used murine models to identify S. aureus traits that support GI colonization. However, the mechanisms governing GI colonization relevant to CA-MRSA are poorly understood, in part due to the use of animal models that rely on antibiotic depletion of gut microbiota to establish colonization. Our recent published and unpublished work adapted an infant mouse model to provide a tractable system relevant to CA-MRSA GI colonization in the community, especially among infants and children. Our preliminary data, obtained using this model, show that weaning is associated with colonization resistance to CA-MRSA. We also show that pore- forming leukotoxins (?toxins?) promote CA-MRSA colonization in weaned mice, but had no effect in infant mice or germ-free adult mice. Given our finding that weaning was associated with colonization resistance to CA- MRSA, a property thought to be conferred by commensal microbiota, we hypothesize that perturbation of commensal bacteria by toxins empowers CA-MRSA to overcome colonization resistance by commensal bacteria. We also established that colonization resistance against CA-MRSA is paradoxically increased in mice that lack adaptive immunity (B and T cells). Given that innate immune cells that shape the gut microbiota during weaning and confer resistance to pathogens are upregulated in such mice, we secondarily hypothesize that innate immunity and the microbiota combine to inhibit CA-MRSA colonization. To test our hypotheses, we will 1) identify commensal species that mediate CA-MRSA colonization resistance in the gut, 2) understand the immune mechanisms that inhibit the CA-MRSA colonization in mice without adaptive immunity, and 3) determine the specific CA-MRSA toxins and interactions between S. aureus and gut commensals that affect bacterial competition. The outcomes of these studies promise to identify bacterial taxa, innate immune mechanisms, and CA-MRSA loci we might manipulate to perturb CA-MRSA colonization. The results will guide future efforts to identify microbiota and cell-type-specific targets for rationally designed therapeutic strategies that modulate colonization. To the extent that the work identifies virulence factors that contribute directly to pathogen transmission, our work will also uncover bacterial mechanisms that could be exploited as targets for dual-action therapeutics.